Bone conduction unit



Jan. 18, 1949.

H. S. KNOWLES BONE CONDUCTION UNIT 3 Sheets-Sheet 1 Filed Oct. 27. 1944 Fig.2

INVENTOR. HUGH 5. KNOWLES WW2.

H/s ATToRN Jan. 18, 1949. s, KNOWLES 2,459,325

BONE CONDUCTION UNIT Filed Oct. 27, 1944 3 SheetsSheet 2 Fig. 6

H/s Arron/vb" Jan. 18, 1949. H. s. KNOWLES 2,459,325

BONE CONDUCTION UNIT Filed Oct. 27. 1944 3 Sheets-Sheet 3 Fig. /5

Relative loudness in decibels frequency in c.p .s.

Fig. 13

INVENTOR. HUGH S. KNOWLES H/s ATTORNEY Patented Jan. 18, 1949 BONE CONDUCTION UNIT Hugh S. Knowles, River Forest, 111., assignor to Zenith Radio Corporation, a corporation of Illinois Application October 27, 1944, Serial No. 560,667.

2 Claims. (01. 179 -107) The present invention relates to hearing aid receivers or earphones of the bone conduction type having a vibratile element arranged to transmit vibrations to the auditory nerve through the bony structure of the head of a user.

It is well known that excessive force between a bone conduction unit and the wearers bony structure causes discomfort and even injury to the wearer. It is therefore desirable to provide a bone conduction unit which is arranged to give the wearer thereof some indication, if and when, excessive force exists between the unit and the wearers bony structure. This feature is desirable, because when one first places the bone conduction unit in position, it is difficult to determine whether too much force is being exerted. Such excessive force in prior art arrangements usually is felt only after a long period of continuous wear, and by that time the discomforts and annoyances of excessive pressure in the form of headaches, impaired blood circulation, reddening of the skin, etc., are sending their warnings.

The term excessive force is used throughout the present specification but it is understood the excessive force existing between the casing and the wearers .bony structure causes excessive unit pressure to exist. In practice the unit pressure is difierent at different contact regions and is equal to the component of the force normal to the persons skin divided by the area in the particular region considered, it being apparent that this unit pressure at such regions depends, inter alia, upon the shape of the mastoid bone when the bone conduction unit is mounted above the mastoid bone. I

Because of the limited electrical output of hear ing aid amplifiers due to space and weight requirements, etc., it is highly desirable to efiiciently convert such electrical output into vibratory energy and then efficiently transfer such vibratory energy to the auditory nerves of the user. Users of bone conduction units quickly find usually by trial and error that converted sounds are louder when the pressure of bone conduction unit on the head of its user is increased. Users of the prior art devices, of the type which are pressed against the users head by the usual resilient supporting headband, oftentimes purposely or otherwise adjust the headband by distorting, bending, etc., it so as to cause relatively high pressures to exist for one or more of the following purposes: of producing enhanced loudness, of producing different tone quality, or of producing a condition whereby the unit is less apt to crawl or slide on the users head.

High pressures produced for these purposes or in many cases resulting inherently from the initial store or factory adjustment are tolerated until the tissue and skin become hypersensitive as they do when such high pressure is continued. Headaches commonly result from the continued application of high pressure and in many cases actual deformity of the adjacent bony structure occurs. Asa result the common practice is to periodically move the bone conduction unit relative to the head, to remove it completely for extended periods, or to adjust the headband by bending, distorting, etc. it after the above described discomforts are observed. If the user is preoccupied he may not introduce such change in operating conditions until the discomfort is relatively acute and after such discomfort has persisted for many hours. Consequently, common practice in the wearing of bone conduction units appears to be that the user initially is subjected to high pressures over and above those necessary for satisfactory transmission of sound and later in order to minimize discomfort it is necessary for the user to reduce such pressure below a magnitude satisfactory for good transmission of sound.

The recognition of these unpleasant effects resulting from sustained high pressure on the users bony structure and the possibility of serious complications resulting from these effects is so widespread among otologists that many of them recommend a bone conduction unit only when an air conduction unit gives very unsatisfactory results.

On the other hand, if inadequate pressure exists between the bone conduction unit and users bony structure transmission is impaired, the intensity of vibration induced in the users body structure is reduced and furthermore the amount of sound transmission as a function of frequency is altered. Consequently, the overall responsefrequency characteristic is impaired and must be altered for optimum tone quality, and in addition the user has lost some sense of loudness. The reduced loudness causes the user to increase the amplifier gain and to frequently overload the amplifier with the result that a high annoyance factor resulting from impairment in the quality of music and from impairment in quality and articulation of speech results.

It follows then that from an acoustical viewpoint, depending upon the meaning ascribed to the term desirable pressure, there are two types of desirable pressure, i. e. (1) a high pressure for excellent sound transmission from the bone conduction unit to the wearers bony structure but too high in pressure for sustained operation because of the resulting undesirable physiological effects produced in the individual and (2) a pressure sufiiciently high for good sound transmission from the bone conduction unit to the wearers bony structure but low enough so as not to cause the resulting undesirable physiological effects in second type of desirable pressure exists whereby the user may be protected against harm of a physi ological nature. subjectively, this second type of desirable pressure varies with different individuals and in bone conduction units arranged to rest above the mastoid bone and the pressurei's within a pressure range resulting from the-application of a force of 6 to ounces to a contact face have" ing an area substantially equal to .8 square inch.

Inasmuch as the desirable pressure rangevar'ies with difierent individuals it is desirable for the purposes of the present invention that the bone conduction unit itself may be adjusted by the user to provide a warning when more than de-' sirable pressure of the second type is exerted. Or, the unit itself may for example be adjusted at the factory, on a statistical basis; to provid'e a warningwhen the pressure exceeds either the pressurec'orresponding to an application force of 20 ounces or an average pressure between the range corresponding to an application of 6 to 20 ounces in a so-called hypothetical normal person.

A third type of desirable pressure may be mentioned. Such third type of desirable pressure is present when the bone conduction unit is mount ed poorly for good sound transmission from the unit to the users bony structure'but is mounted to rest with great comfort on the users person. This condition of operation results in the disadvantages mentioned previously. It will be readilys'e'en'that applicants mode of operation of the bone conduction unit for realizing the second type of desired pressure corresponds toa-n adjustment intermediate the adjustments for obtaining the first and third types of operation mentioned above. There has existedfor a long time the need'for a' headband supported bone conduction unit which would automatically warn the user when this second type of operation was not present.

As will be set forth presently this condition is realized by the user when the bone conduction unit satisfactorily engages the head with suifi cient high pressure for suitable sound transmis sion' and perhaps tone but with suificiently low pressure so as not to cause loss of sound .trom the unit. This want has particularly been pres ent in bone conduction arrangements which are supported other than by the users hand and by means which exert a substantially constant continuous force over a relatively long period of time such as headbands or other types of harness. Al'- though the bone conduction arrangement include ing the bone conduction vibratile member and headband may be set so that theuse'r needn'ot determine at a later time the pressure which is excessive, excessive pressure 'may later be developed after the arrangement is displaced from its initially adjusted positionthroughnormal or abnormal activities of the user. Such excessive pressure may be produced through inadvertent or careless bending or adjustment of the headband, or from wearing a hat placed'too tightly on the headband while the instrument is' being worn. i

It is therefore an object of the presentir'lveritionto provide a bone conduction unit which is arranged to give the wearer thereof someindic'a tion when and if excessive force exists between the unit and the wearers bony structure.

A further object of the present invention: is to duction type having a casing which is arranged rovide animproved bone conduction arrangement which provides an adjusted warning point or pressurejwhi'c'h should notice exceeded. H

Still another object of the" present invention is to provide an improved bone conduction arrangement which warns the wearer instantly, by loss of sound, whenever force of the arrangement against the wearers bony structure exceeds an. {amountlikel-y to cause excessive discomfort after -continued use.

Yet anotherobj'ect of the present invention is to'provide an improved bone conduction arrangement which causes the vibratile member thereof to be inoperative for the efficient transmission of acoustical energy when pressure is higher than that for which the arrangement is adjusted.

It is also-an object of the present invention-to provide an improved earphone of the: bone con to move with respect to a relatively stationary supported magnet assembly, thedriv'in'g motor being especially arranged so that -ex'cessivegpr'es sure of thecasingagainst the head of a 'userzdur i ng soundreception causes loss of movement or.

the casing.

It is further desirable that, when oncea-warn' ing has been given to the wearer inresponse-to excessive pressure between the unit and the wearers bony structure, such warning b'e'di'scon tinued after the pressure isagainadjusted tofa lower normal operating pressure without mar -lip;

ulating' elements in theum't' such as by' shaking the unit,- etc. a

It is therefore a further object of the present to operativeness when and if the excessive pres-- sure is removed.

The features of the present invention which are believed to be novel are set forthwith particularity in the appended claims.

operation, together with further objects and advantages thereof may best be understood by reference to the following description taken cone.

Fig. 2 shows an arrangement mounted in op erative position on a person and utilizing a principle of the present invention;

Fig. 3 shows an exploded view of part of the apparatus shown in Fig.- 1;

Fig. 4 shows an assembled view shown in Fig. 1; 7

Figs. 5 and 6 are sectional views taken respectively on lines 5-5 and 6-6 of Fig.4;

Fig. '7 shows parts of a modified structure embodying the present invention;

. Fig. 8 is a sectional view taken alongline-B-AB of Fig. '1 i of apparatus Fig. 9 is a sectional View taken-along line 9 9 of Fig. 7; h I V Fig. 10 is a sectional view of a portion of Fig. 7; p Fig. 11 is a perspective view of the apparatus of Figs; 1 and 2 arranged on a person for the The present invention itself, both as to its organization and manner of Fig. 14 being an elevational view of the arrangement; and

Fig. 15 shows certain characteristics of apparatus embodying the present invention.

The particular bone conduction arrangement shown in Figs. 1-6 and 11 is of the type adjusted on a statistical basis so as to allow a so-called normal person to obtain the second type of operation mentioned above with the assurance that he will be automatically warned when operation is of the first and third types also mentioned previously. 7

The modified arrangement shown in Figs. 12- 14 is of the type which any particular user may adjust without taking the bone conduction unit apart whereby the unit may be adjusted accurately to his own particular needs to obtain the second type of operation mentioned above with the assurance that he will be automatically warned when operation is of the first and third types.

In Fig. 1 the bone conduction earphone or receiver designated generally by the numeral I and embodying a principle of the present invention, comprises a casing II, a magnet assembly I2 and a movable armature I3 arranged to' be fixed to cover I4 of easing II, a magnet assembly support bar I5 arranged to be fixed on magnet assembly I2 by nut I6 (Fig. 3) and arranged also to screw threadedly receive a pair of yoke supporting pins IT, IS which extend respectively through a pair of oppositely disposed slightly oversized openings I9 and 2B in casing II, the openings I9 and 25 being made oversized to allow vibratory movement of casing II relative to the pins I1 and I8.

As illustrated in Figs. 2 and 11, the casing II is arranged to press against a suitable portion of the users head, for example, the mastoid bone, by mounting the composite earphone, or receiver ID, on a suitable headband 2| having a resilient yoke member 22 which partially surrounds casin l I (Fig. 4) and resiliently and frictionally engages the enlarged body portions of pins [7 and ,I 8 on an axis corresponding to line 5-6 in Fig. 4, the yoke member 22 having a pair of oppositely disposed apertures 23 and 24 and being frictionally mounted on one leg of L-shaped member 25 by means of rivet 26. The other end of L shaped member 25 is frictionally mounted on one end of headband 2! by means of rivet 21. The casing I I is thus mounted, for adjustment relative to a persons mastoid bone, not only for rotative movement about the axis of pins I1 and I8 but also for rotative movement about the axis of rivet 2E and for rotative movement about the axis of rivet The casing cover I4 arranged to contact a region on the wearers skin near the wearers mastoid bone is made concave along its longitudinal axis only as shown in comparison in Figs. '7 to 9 and is pressed into suitable contact with a region near the wearers mastoid bone by forces resulting from the stressed resilient headband 2 I, which forces are transmitted through the frictional connections previously described and bearing the reference numerals 2B, 29, 3D and 3I. This particular shape of the contact surface of cover I4 and frictional connections 28, 29, 3B and 3| allows the cover I4 to satisfactorily engage the wearers head and reduces the tendency of easing cover I4 to crawl off the mastoid bone and further controls the low frequency response by giving more uniform compliance between the users head and casing II. This term, compliance, is used in its generally accepted meaning" in the acoustical engineering art as set forth in the book Applied Acoustics, by Olson and Massa (2nd edition published by P. Blakistons Son 8: (30., Inc., Philadelphia, 1939).

With this arrangement, the force exerted by the casing I I on the users head is dependent upon the resiliency of band 2! or forces exerted on such band, for example, by the wearers hat.

Another feature of the particular headband construction shown in Figs. 2, 4, and 11 is that a receiver, or earphone It, of the bone conduction type is held in operative position with the headband extending over the top of the users head so that the users normal activities influence in a relatively small degree the force of casing I I containing magnet assembly I2 on a suitable portion of his head.

The magnet assembly it, including the armature I3 is shown in an exploded view in Fig. 3, and in sectional views in Figs. 5 and 6.

In Fig. 3, the armature I3 of magnet assembly I2 is disposed within, and formed integrally with,

' a surrounding flexible member 32 having a pair of oppositely disposed ears 33 and 34, the armature 53 being joined to the surrounding portion 32 by portions til and SI of predetermined resiliency. The armature I3 is fastened to the casing cover I4 by means of a pair of countersunkhead screws 35 and 36 (see Fig. 4) passing through casing cover I4 and engaging the tapped cylindrical studs 3! and 38 mounted on armature I3 the casing cover I4 in turn being fastened to casing II by means of countersunkhead screws 62 S3 and 34 (Fig. 4).

The armature I3 and the integrally formed surrounding member 32 are supported on the magnet assembly I2 by means of a pair of screws 39 and 40 (Fig. 6) passing respectively through apertures M and 42 in cars 33 and 34, the screws being threadedly held in ears 43 and 44 of top plate 45 of the magnet assembly I2. Spacers or shims 46 and 41, interposed respectively between ears 33 and 43 and between cars 34 and 44, hold the movable armature I3 in predetermined spaced relationship to core member 48. The thickness of these shims are of importance, as will be seeen later, in determining the force necessary between casing II and the wearers mastoid bone to cause armature l3 to be inoperative for transmitting intelligence.

The strap or bar I5 is fastened by nut IE to the bottom of base plate 50 of magnet assembly I2, Bar I5 has a pair of ears 5| and 52 each having a tapped hole therein for engagement with threaded pins I1 and I8 (Figure 1) arranged to project through openings I9 and 20 in casing II.

These previously described parts of magnet assembly I2 are mounted on the base plate 50. In addition, magnet coil 54 is fixedly mounted on base 50 by passing the reduced threaded portion 56 of core member 48 through the insulating annular coil form 65 having mounted thereon the magnet coil 54, and threading nut I6 on threaded portion 56 so as to clamp bar I5 between base plate 50 and nut l6.

Permanent magnet pole pieces 57 and 58 (Figs. 3 and 5), preferably of the type having aluminum as one of its constituents, are clamped in spaced relationship between base plate 50 and top plate 45 by means of a pair of countersunkhead screws 59 and 50 passing respectively through apertures GI and 62 in the base plate 50, through clearance openings 63 and 64 in magnets 51 and 58 and screw threadedly engaging top plate 45 in tapped holes 63 and 61, the coil form 65 being armature l3 results.

also clamped between base plate 50 and. top plate.

It is thus seen that the armature l3 joined t the resilient surroundin member 32 by integral resilient portions 60 and 6! (Fig. 3) is mounted:

and has mounted thereon the spaced tapped cylindrical portions 3'! and 38 for fastening cas ing cover 14 to movable armature I3 whereby casing H follows the movement of armature [3.

One of the important features of the present invention is that the armature l3 is arranged to transmit suitable low intensity vibrations to the casing I I without the production of any looking" or freezing action or without the distortion of audible sounds resulting from such vibrations when normal forces areexerted between casing H' and the wearers mastoid bone but does produce cessation or diminution of movement of movable armature with respect to its field structure when and if slightly more than normal forces are exerted between the casing H and the wearers mastoid bone. In other words, even though the armature it may touch the core member 48 in the inoperative position or" armature [3, the tension forces developed in connecting members 60', v

65' and surrounding members 32 increase 'more than proportionally with their displacement with the net result that the restoring forces developed are always greater thain the magnetic forces existing between core member 48 and armature il3 and henceno locking or freezing of the For this purpose, member 32 and integrally formed members 66 and 6t (Fig. 3) connecting the armature 13 to its ring shaped flexible supporting member 32- made of such dimensions and of such'material that nor-j mal forces exerted between the users head and casing t] cause a relativelysmall amount of movement of armature it. But, the portions '60 and Biand ring shaped member 32 are sufii ciently flexible so that armature it and casing ll produce sound vibrations without distortion When currents of sound frequency and of suitable intensity flow through coil 54 from terminals similar to terminal 68 in Fig. 5 which tert minalshave a projecting contact 69 arranged to make contact with an electrical plug type contact frictionally held in opening 10 in casing H.

When currents of voice frequency flowfrom a pair of contacts, similar to contact 68 inFig. '5,

and through coil .5 3, magnetic flux is setup in coil 54. A first portion of such flux'traverses core member 48, armature i3, top-plate; 4'5, magnet 51', bottom plate 50, and returns to core member 48. A second portion of such flux traverses core member 48, armature 13, top plate 45', magnet 53, bottom plate 50, and returns to core member d8. Both of these first and second mentioned courses of the flux are effective jointly to cause movement of armature l3 and casing ll in re- Y sponse to current flow through c0i1i'54. The mag nets 51 and 58, of course, are poled such that they produce a magnetic field in core member f ltisub-v stantially equal to twice that produced byvon'e magnet alone.

In the preferred embodiment of thepresent invention, the air gap distance between armaa ture l3 and pole piece 481m of the order off ;003 inch to .005 inch-When no force exists betweencasing H and the wearer's mastoid bone. This air gap'distanoe. is of such amount andthere-silk iency of ring. shaped member 32' and'connecting. portions 60 andfil are such that-when forces: above a. predetermined magnitude are. present between casing I! and the wearersmastoid bone the armature i3 is displacedfrom operating position and is inefiective to transmit variations-Produced by currents of voice frequency flowing in coil 54*. When the abnormal force which pro-f duces such abnormal displacement of the arr-na ture is removed, the armature is automatically eifective to resumeits normal operation of vibrating in .accordance' with currents of voice frequency in coil 543 In other words, so'long. as the armature i3 transmits vibrations to. thewearers mastoid bone, the wearer is automatically: ad'-: vised that normal forces exist between casing H and his mastoid bone but, when such- ;torce: exceeds a predetermined safeforce, the weareniis automatically, informed by loss of sound that such force is excessive. 1 Excessive force between the casing H and the wearers mastoid" bone oftentime results from the wearers hat worn so as to produce relatively large stress in the head band 2'! which contacts the wearers hat; In such case, oftentimes, the resilient headband '21:

is depressed into the space bearing .the'reference numeral 12 in Fig.2 and accordinglythecasing H- is pressed too hard againstlt'he wearers' mas toid bone. When such condition exists :the wearer is automatically informed by loss of sound that such condition exists.

The particular manner of mounting. the-armature 13 withthe resilient portionsfl'll and" 61- (Fig. 3) symmetrical abouttheline GFBJJin Fig. 44 allows the bone conduction unit proper tobe used effectively even though the unit. may: be inadvertently or carelessly mounted in position on the users head without. excess. force: existing between casing H and thewearerssmastoid bone. Moreover, such portions 6|]; 6| and ring 32 are of such size and shape so asto bend at small'am'pli tudesof vibration of armature l3 and to'b'e' predominantly tensioned when armature [3 tends to vibrate with increasing amplitude. Thus, the manner of mounting armaturel3 with vthe symmetrically and integrally formed" resilient por- 1 tions 60, 61 and 32' not only assures operation when the bone conduction unit proper is-slightly improperly positioned: without the introduction of excessive pressure but also assuresconversion of electrical energy into vibratory energy of low Furthennore, cessation of vibration of the,

armature I3 is prevented in use, when normal forces exist between the casing H andwearers. mastoid bone, not only by {causing proportionally larger. tension forces to appear in.resilient portions 6 0, 6| and 32 when armature I3 is displaced.

objectionably large distances fromitsstatic position, but also due to the: fact that increased eddy currents flowing in the. unlaminated armature I.3 cause a greater demagnetization ofthe field from core 48 as armature l3 approaches core 48; Also since the magnetic circuit is of such cross sectional area forlthe flow of magnetic: fluxthat saturationand consequent reduction u and the of permeability results when the armature I3 tends to move too close to core 48.

In the normal range of movement of armature I3 within a restricted low amplitude range, the armature I3 is displaced a distance linearly proportional to the intensity of current flowing through winding 54. This is accomplished in some degree by suspending the armature I3 by means of the resilient members 60, GI and 32 such that proportionally larger tension forces arising in such members oppose the proportionally larger magnetic forces on armature I3 as it approaches core member 48. That is, the increasing positive stifiness of the composite armature I3 as it moves toward the core 48 is substantially offset by complementary, negative resistance arising from increasing attraction between the armature I3 and core 48.

In Fig. 7 the magnet assembly I2 is mounted on the cover I4 in the manner described above but the assembly I2 is connected to a headband through a universal connection 74 shown in greater detail in Fig. 10. One part I5 of the universal joint is rigidly connected to the field structure of assembly I2 by means of a screw threaded connection and extends through a clearance opening IS in the back of a modified casing 11, the cover I4 being secured on casing IT by means of screws disposed in tapped holes 78 in casing TI. The other part I9 of the universal joint may be connected to a headband.

In the various arrangements shown herein the casing II or 11 is made to vibrate with respect to the armature assembly I2 and for that reason oversized holes I9, and I6 are made in the casing so as to allow freedom of movement. In order to prevent foreign materials entering the unit and impeding the movement of armature I3 or assembly I2 with respect to casing I I resilient washers such as shown in Fig. 13, resiliently engaging members I'I, I8 may be employed to form a closure member for the openings I9, 20.

Figs. 12-14 show a modified arrangement incorporating means for adjusting the bone conduction unit to the particular needs of an individual in a manner so that the individual may be warned of the existence of an adjustable predetermined force existing between casing II and his bony structure. That is, means are provided for adjusting the unit to difierent predetermined forces at which the unit becomes inoperative. Corresponding parts in the various figures herein are allocated identical reference numerals and the additional structure in Figs. 1214 is described now.

The tension on armature I3 is adjusted by compressing leaf spring 88 by adjusting the position of adjusting screw 8| held screw threadedly in rigid yoke member 82 which extends around the magnet assembly and abuts armature I3. It is apparent that it is in the scope of one skilled in the art to permanently fasten the yoke member 82 to armature I3 by conventional means, if desired. Leaf spring 80 having its ends abutting base plate 5!! and its recessed center portion enaged by the end of adjusting screw BI is guided during its adjustment and held in its adjusted position by means of pins 83, 84 which extend through cooperating apertures in the spring 88. It is readily apparent that the spring 80 is dis posed between the relatively stationary magnetic field structure and its cooperating armature I3 and when compressed causes the armature I3 to move closer to the core member 48. Further- 10 more, the greater the compression of leaf spring 80, the lesser the force between casing II and the wearers mastoid bone need be to cause armature I3 to be inoperative to transmit vibratory energy to the individuals mastoid bone.

The position of adjusting screw SI determines the force necessary to cause armature I3 to be inoperative and may be suitably indicated on casing II by providing suitable cooperating scale on casing II and indicia 86 on screw 8|. The designation H on casing I I in Fig. 14 signifying that part of scale 85 which corresponds to high forces necessary to cause armature I3 to be inoperative for the transmission of vibratory energyand conversely, that part of the scale bearing the designation L corresponds to low forces necessary to cause armature I3 to be inoperative for the transmission of vibratory energy. The region marked Hfmay correspond to a force of 20 ounces and the region marked L may correspond to 6 ounces with each scale graduation corresponding to a force of two ounces.

Fig. 15 shows the response-frequency variation of vibratory energy transferred subjectively to an individual when as in curve 81 the bone conduction unit is suitably mounted for obtaining the second type of desired operation mentioned above wherein the casing I I is pressed against the individuals anatomy with a force sufiicient for good sound transmission and good tone quality and not too excessive to produce inoperativeness of armature I3; and, when as in curve 88 the bone conduction unit is mounted for obtaining the so-called third type of operation mentioned above wherein casing II is pressed against the individuals anatomy with a force much less than that necessary to cause inoperativeness of armature I3 and for great comfort.

It is readily apparent from the shapes of curves 81 and 88 when drawn into comparison, that the third type of operation mentioned above is automatically realized by the individual who may provide the necessary adjustment by bending the attached headband 2| etc. in order to obtain enhanced loudness and enhanced overall tone quality.

While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects, and. therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of this invention.

I claim:

1. A bone conduction unit including a driving structure and a casing surrounding said driving structure, said driving structure including a magnetic field structure, a pole piece supported in said field structure and an armature supported on said magnetic field structure in normally spaced relationship to said pole piece to permit vibration of said armature in response to those variations in density of magnetic flux from said pole piece corresponding to audio signal currents impressed on said field structure, said armature being connected to a first region on said casing to transmit vibrations to said casing, a spring element interposed. between a second region on said casing and on the side of said field structure opposite said armature and positioned to exert a force on said field structure in the direction of said armature, and an adjusting element associated with said spring to vary the force exerted on said field structure. I

acaaczs 21. {a hearing: aid; oi' the. bone conduction type: whichreqm' res apressurea contact with a users amatcmyto. induce thesensation 0t hearing efli-- rimm-y; said hearing. aid inciuding a driving. strum ture and a. casing surrounding said driving struc ture; said: driving structure including a magnetic: field structure, a pole piece: supported in saictfield structure and an. armature supported on saint magnetic field, structure in normally spaced relationship to said pole piece to permit vibration of said armature in responseto. those variations in density of magnetic flux-fromsaid: pole piece correspondingto audio signal currents. impressed; on. said field: structure, said armature being conriectect tov a first region on. said casing to transmit vibrations to said, casing, a spring element interposed between asecond: reg-ion on said casing and: on the; sideof saidifield structure oppcs-iteside armature and positioned to exert. a

force: onsaid field structure: in the direction of saict armature tending: te effect a condition of ink operatability of said driving: structure, means for holdin'grsaid hearing aid forceful contact with the usersanatomy, said. means being mechanicalty coupled to-said' field structure throughone or more: apertures: in said? casing to cause such.

holding force to be exerted. on saiclfield structure in aiding relationship-tame spring forcewhereby said! fietct structure is urged in forceful contact with said armature to cause a condition 'of-in-= op'erativeness when the combination of the spring and. holding forces is: excessive, and, an adjustingelement associated with said spring to, vary'the force exerted; rm said field structure whereby inoperability maybeadjusted to occur at a predetermined excessive holding pressure.

HUGH s KNOWLES.

REFERENCES crrnn The following references are file of this patent:

UNITED STATES PATENTS c of" record in the i 

